Basically there are 2 kinds of sandpapers available for orbital sanders:

1) regular paper that is fitted using a clamping system on the sanding base, this is the cheapest paper;

2) and paper with a velours backing that can be used if the sanding base has Velcro strips. This paper is more convenient to use, but is also more expensive. Some sanders have both clamps and Velcro, so you can choose which kind of sandpaper to buy.